Amazon announced this week that come May its customers will be able to buy Kindle Fire apps and some other goodies with a new virtual currency called Amazon Coins. And to jumpstart the program, they will give away millions of dollars worth of coins. The giveaway is a great way to get started, but it points to a problem with using the word "currency" to describe what Amazon has created. Calling Amazon Coins a virtual currency suggests that it will be a widely accepted, independent store of value that you can easily convert to another currency. But from the few details Amazon has given, there's no reason to think this will be anything other than yet another in-house system of credits. If Amazon Coins are currency then so is the card I use to get on the subway in New York, because it works the same way—you load money onto an account. But once its there, the subway system is t...